DEEP SEISMIC STRUCTURE OF THE EARTHS CRUST ALONG THE BALTIC SEA PROFILE

In summer of 1989 the Institute of Oceanology of the Russian Academy o f Sciences (Moscow) for the first time used a new large volume airgun for deep seismic sounding (DSS) in the Baltic Sea. The experiment was carried out during the 23-rd cruise of R/V Professor Shtockman. The wa ve-field analysis showed clear P and S wave arrivals, including Moho r eflections. The Moho reflections were distinctly observed at distances exceeding 100 km. The P waves can be followed to a maximum distance o f 368 km. Two-dimensional raytracing modelling of the data revealed a pronounced topography of the Moho along the BALTIC SEA profile: a depr ession in the middle of the profile at a depth of 45 km is bounded by 2-3 km steps and uplifts on both sides. Reflections off a sub-Moho bou ndary at a depth of 60 km have also been recorded. The upper mantle ve locities vary between 7.8 km/s under the depression to 8.1 km/s to the side of it. The crustal P waves velocities define three layers: 5.8-6 .3, 6.3-6.7 and 6.9-7.1 km/s. All crustal velocity boundaries bend ove r the Moho depression and reach their deepest points in the central pa rt of the profile. The velocities in the crust also change laterally, reaching their minimum within the central depression. The crustal thic kness along the profile varies notably between 38 and 45 km. A compari son with the FENNOLORA and the BABEL B profiles indicates that the Moh o depression found along the BALTIC SEA profile correlates with simila r depressions on the other profiles.